I. Field of the Invention
The present invention relates generally to a method of operation of a nickel-hydrogen battery, and more particularly, to the method of recharging a nickel-hydrogen battery at a temperature which is substantially lower than the temperature at which discharge is performed.
II. Description of the Prior Art
It is well recognized that optimum performance for nickel hydrogen spacecraft batteries requires that their temperature be controlled in the range of +20.degree. C to -10.degree. C. Higher temperatures lead to lowered capacity due to the early on-set of the parasitic electrolysis reaction, e.g., EQU 2OH.sup.- =1/2O.sub.2 H.sub.2 O+2e.sup.- ( 1)
Reaction (1) has been recognized in the prior art to reduce capacity as the operating temperature of the battery is increased. A key to the present invention has been the recognition that the critical temperature is that at which the battery is recharged and reaction (1) competes with the normal recharge reaction, e.g., EQU Ni(OH).sub.2 +OH.sup.- =NiOOH+H.sub.2 O+e.sup.- ( 2)
Lower temperatures on the other hand have been found to lead to batteries which fail to operate. The reasons for this failure, prior to the present invention, were unclear as the freezing point of the electrolyte (-61.degree. C.) is well below the minimum operation temperature. The second key to the present invention has been the recognition that the lower temperature limit is due to a limitation in the ability of the battery to sustain high rate discharge required in satellites situated in a geosynchronous orbit (e.g., .about.C/1.5) as opposed to the lower rate recharge (e.g., .about.C/10).
Typical of the prior art as it relates to charging and discharging nickel batteries is U.S. Pat. No. 4,680,241 to Dyer. The Dyer patent concerns a method for partially or fully restoring the lost capacities of nickel batteries. In this instance, a nickel battery is cycled at least 10 times, with each cycle including a discharging step during which the capacity achieved at the end of the previous cycle is reduced by at least 5 percent, and a charging step. The charging rate employed during the charging step is greater than about C/10 per hour. Moreover, while the ratio of the amount of charge delivered to the battery during the charging step of each cycle to the amount of charge withdrawn from the battery during the previous cycle is greater than one, this ratio is chosen so that the temperature of the electrolyte of the battery does not exceed about 30.degree. C. It is clear, however, that the Dyer patent does not address the particular problem which the present invention is intended to solve. It was in light of the state of the technology as just described that the present invention has been conceived and is now reduced to practice.